The aims of this study were to cultivate cos lettuce (Lactuca sativa L. cv. Cos Lettuce) using quantum dot (QD) sheets for wave conversion and to clarify the advantages and disadvantages of this technique for plant cultivation with artificial lighting. Blue LED light was wavelength-converted to green (G-QD) or red (R-QD) light by QD sheets and used for cultivating cos lettuce. Then, lighting with spectral photon flux density distributions similar to those of G-QD and R-QD was designed using green or red LED and blue LED (G-LED and R-LED), respectively, and used for cultivating cos lettuce. Plant growth at 14 days after treatment revealed that the shoot fresh weight was greater under G-QD compared with G-LED, perhaps because the amount of integrated light reception was greater in G-QD owing to the difference in the photosynthetic photon flux density's spatial distribution. Plants showed similar growth levels and morphologies under R-QD and R-LED lights. Plant growth is affected by the light environment, such as spectral photon flux density distributions and its vector field, regardless of the light source. However, the prototype QD sheets used in the present study had low conversion efficiencies and wide directivity angles. If QD products are developed that utilize the advantages of high monochromaticity and long life for plant cultivation at low costs, then they may become widely used.